The Multiple Sleep Latency Test (MSLT) was performed twice after 8 h and after 4 h of night sleep in 15 healthy young subjects (mean age: 23 y). Seven subjects could be regarded as morning, 8 subjects as evening types. After 8 h of sleep significantly more evening types napped at 08.00 hours and at 12.00 hours. Evening types rated themselves more sleepy on an hourly administered visual analogous scale (VAS). Sleep onset latencies (SOL) decreased, and the amount of Stages 1 and 2 increased in all subjects dependent on the sleep restriction condition. No significant differences between morningness and eveningness concerning SOL and structure of nap structure could be observed. After 4 h of sleep there was a marked increase in subjectively rated sleepiness during the morning hours in both groups.